1. Field of the Invention
This invention relates in general to amplifier designs and, in particular, to an amplifier design which is useful with a high Q magnetic head.
2. Description Relative to the Prior Art
In the playback of signals from magnetic tape, either of two general types of playback circuits may be employed, to wit, a current mode circuit or a voltage mode circuit:
A current mode circuit is one in which the current produced by a magnetic head is virtually flat as a function of frequency. That is, as the frequency of a playback signal increases, thereby causing the voltage produced by the head coil to rise, so too does the inductive reactance of the coil, resulting in the current being flat with frequency. At low frequencies, because the voltage induced in the head coil is extremely low, the current will be limited by the resistance of the head coil winding. To counter the fall-off of current at low frequencies, the head coil resistance is made as low as possible, typically by using relatively heavy-gauge coil wire. Attendantly, this means larger head structure, and diminished performance with respect to high frequencies, caused by wire skin effects and leakage. Current mode circuits employ low input impedances.
A voltage mode circuit, on the other hand, employs a high input impedance and results in the playback signal from a magnetic playback head having a response that increases with frequency, i.e. is d.phi./dt dependent, where .phi. is the magnetic flux in the head core.
In the design of a magnetic playback head, it is customary to place as many turns on the head as possible without causing the resonant frequency of the head to fall within the passband of the range of frequencies to be handled by the head. For example, if the bandedge of interest is 3 MHz, the turns on the head, as designed, will be such that the resonant point will fall above 3 MHz. If the resonant point were to fall below 3 MHz, the number of turns typically would be decreased to raise the resonant point above 3 MHz, thereby sacrificing signal output, but avoiding the voltage peaking that would occur at resonance.
Since, in a voltage mode usage for a playback head, the signal output is d.phi./dt dependent, an equalizer circuit is usually employed to flatten the voltage versus frequency response curve. Ordinarily, for most prior art heads, the job of the equalizer is not stringent. That is, a typical prior art playback head has a Q of about 1 and, thus, its peaking at resonance is minimal, and easily compensated for by an equalizer.
The present trend in the playback of magnetically recorded signals is to use relatively high Q heads, such as ferrite heads. While such heads are highly efficient, and exhibit little change in inductance with frequency, i.e. head output voltage increases extremely linearly with frequency, they do exhibit fairly sharp peaking at their points of resonance. Since the inductance of, say, a ferrite head can vary within 20 percent of a manufacturer's target for inductance, resonance for a ferrite head may fall below the upper bandedge of interest. This, coupled with the fact that inductance-change (decrease), resulting from head gap-depth decreasing as a result of head wear, means that the job of the afore-mentioned equalizer is extremely difficult to implement.
(To be noted is that, whereas the concept of U.S. Pat. No. 3,310,637 is directed to use of resonance peaking well within the passband of interest -- and current mode operates as per its FIG. 5 -- the present invention primarily concerns the use of resonance outside the passband of interest but, as will appear below, provides, among other things, toleration for shifting of the resonance point to within the passband of interest.)
To nullify the effect of sharp peaking at resonance in a high Q head, thereby facilitating equalizer design, it would appear that a resistance, equal to about the inductive reactance of the head coil at resonance and placed in parallel with the head coil, would lower the Q of the head. That way, at low frequencies, the resistance would not load the head; and at high frequencies, i.e. those near resonance, head response damping would be effected, as desired. Indeed, since ##EQU1## See, for example, Electronic and Radio Engineering, Frederick E. Terman, page 52, e.g. 3-17, McGraw Hill Book Co., NY, 1955) as a shunt resistance is placed across the head, the resulting R.sub.P (i.e. the parallel combination of R.sub.P and the shunt resistance) equals X.sub.L, thereby causing Q = R.sub.P /X.sub.L to equal 1. Such a tack however, while working well to damp response at resonance, causes the head SNR to degenerate appreciably. This is because a resistor, in parallel with the head coil, and equal to X.sub.L at resonance, causes the signal to decrease in proportion to the value of the resistor whereas the noise decreases in proportion to the square root of the resistor.